Some conventional file system for a computer temporarily stores files, which have been read from an external storage apparatus such as magnetic disk devices, in a memory, such as a cache memory, within the computer in order to increase the speed of access to the files stored in the external storage apparatus.
As the files which are access targets are retained in the cache memory, when access to the same files occurs, the access can be processed within the computer. Consequently, it is possible to reduce overhead required for the computer to access the external storage apparatus again.
However, since the cache memory has a limited capacity, the cache memory cannot always retain the same files. So, when it is necessary to fetch files, which are not retained in the cache memory, in the cache memory and if an unused area for fetching such files does not exist in the cache memory, the unused area is formed in the cache memory.
For example, if there is no unused area in the cache memory at the time of I/O reception when executing I/O (input/output) processing such as read processing or write processing, a control method of destaging data in the cache memory based on the number of accesses to files and access intervals is suggested (see Patent Literature 1).